This clinical trial studies fludeoxyglucose F 18 (FDG) positron emission tomography (PET)/computed tomography (CT) in predicting chemoradiation therapy (CRT) failure in patients with stage IIIA non-small cell lung cancer (NSCLC). Diagnostic procedures, such as FDG PET/CT, may help predict CRT failure. Comparing diagnostic results during CRT may help doctors predict a patient's response to treatment and help plan the best treatment